Note that Qt 4 will be installed by the command above (it is required by QCA anyway), but you can build your own from qt-copy in KDE svn, which will include several fixes not yet in a released version of Qt as well as giving you the option of compiling a version with debugging information. If you do this, the system installed QCA (and any other system libraries that use Qt 4) should work without a problem in your local KDE build.

All of the dependencies in kdesupport can be installed from the [extra] repository. These will be fine for building from the KDE stable branch, but some may be too old for building KDE from trunk.
pacman -Sy automoc4 akonadi eigen taglib soprano strigi qimageblitz phonon
To build the latest versions of these instead, see building kdesupport

Note that there may be other optional dependencies of KDE modules, but almost all of these can be satisfied by [extra], [community] or the AUR. The missing optional dependencies will be listed when configuring a package with cmake. To search and install from all the main repositories and the AUR at the same time, you can use Yaourt.